Provide better error message for invalid 'reference' setting
Provide better error message for invalid 'reference' setting

When e.g. specifying an invalid 'reference' value for an IMAP server to
a root folder that does not exist, we would previously have crashed with
a nonsensical and non-intuitive error message (trying to address an
element of a NoneType).

This will also raise an Exception (which should be ok, given that this
is really a misconfiguration on the user side), but it will explain to
the user WHY it exited.

Signed-off-by: Sebastian Spaeth <98dcb2717ddae152d5b359c6ea97e4fe34a29d4c@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <347709267ddd1958560c4c317ea3fd613f10a719@laposte.net>

frioux/offlineimap,frioux/offlineimap